Question

For each of the salts on the left, match the salts on the right that can be compared directly, using K_sp values, to estimate solubilities.

(If more than one salt on the right can be directly compared, include all the relevant salts by writing your answer as a string of characters without punctuation, e.g, ABC.)

	1.	lead iodide		A. PbS
	2.	cobalt(II) carbonate		B. Fe(OH)2
				C. Cu(OH)2
				D. BaSO4

Write the expression for K in terms of the solubility, s, for each salt, when dissolved in water.

lead iodide

cobalt(II) carbonate

Answer 1

Lead Iodide has the formula of PbI2 and Cobalt(II) Carbonate has the formula CoCO3

PbI2 -----> Pb(2+) + 2I-

Let the solubility of salt be s, then Ksp expression for PbI2 = s(2s)^2 = 4s^3

CoCO3 -----> Co(2+) + CO3(2-)

Ksp expression for CoCO3 = s^2

Hence lead iodide will map to B and C i.e. Fe(OH)2 and Cu(OH)2 all having Ksp of 4s^3

Hence cobalt (II) carbonate will map to A and D all having Ksp of s^2

b)

PbI2 -----> Pb(2+) + 2I-

Ksp = [Pb+2][I-]^2 = (s)(2s)^2 = 4s^3

CoCO3 -----> Co(2+) + CO3(2-)

Ksp = [Co(2+)][CO3(2-)] =s(s) = s^2